1. Field of the Invention
The present invention relates to a solid-state imaging element and a solid-state imaging apparatus.
This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2012-120629, filed May 28, 2012, the entire contents of which are incorporated herein.
2. Description of Related Art
As one of auto focus (AF) methods to be performed using an imaging signal acquired by an imaging element, a contrast method (hill climbing method) is known (for example, see Japanese Unexamined Patent Application, First Publication No. 2010-256924). In the contrast method, the imaging signal is acquired while a lens is moved (extended or retracted), a position at which its high-frequency component (contrast value) peaks is determined to be a focusing position, and the lens is moved to the position.
A processing procedure of a specific AF process will be described with reference to FIG. 14. FIG. 14 is a graph illustrating a relationship between a position of a lens and a contrast value known in the related art. In the illustrated graph, the horizontal axis represents the position of the lens and the vertical axis represents the contrast value. When the AF process is started, an increase/decrease in the contrast value is first determined by driving the lens in an arbitrary direction (Y1; direction determination process). Subsequently, the lens is driven in a direction in which the contrast value is increased based on the direction determination result and the contrast value moves from an increase direction to a decrease direction, so that it is determined that the lens has passed the focusing position (Y2; focusing position passage determination process). Thereafter, the AF drive process ends by restoring the lens to a maximum position (peak position) of the contrast value, that is, a focusing position (Y3; focusing position restoration process).
Next, processing timings of the imaging process and the AF process will be described. FIGS. 15A and 15B are timing charts illustrating the processing timings of the imaging process and the AF process known in the related art. FIG. 15A is a timing chart illustrating the timing of the image process, that is, the drive timing of the imaging element. When the imaging process is performed in the illustrated example, an operation of resetting the imaging element is performed after the AF process has been completed. Subsequently, an exposure/accumulation operation is performed. Subsequently, a signal is read from the imaging element. Thereafter, the process waits until the AF process is completed.
FIG. 15B is a timing chart illustrating the timing of the AF process, that is, the timings of contrast evaluation and lens drive processes. In the illustrated example, the contrast evaluation is first performed, the focusing position is subsequently calculated based on a signal obtained in the contrast evaluation, and a subject is focused by performing the lens drive process based on the calculation result. Thereafter, the AF process is performed after the imaging process has been completed.
Because the process is performed using a signal output by the imaging element in the AF process using a contrast method, it is difficult to acquire a signal necessary for the contrast evaluation while the imaging element performs the imaging process. Thus, as illustrated in FIGS. 15A and 15B, the AF process (the contrast evaluation and lens drive processes) is performed until the reset process of the imaging element is started after a process of reading the imaging signal from the imaging element in the imaging process is completed.
In addition, because an auto exposure (AE) process is also performed using a signal output by the imaging element, it is difficult to acquire a signal necessary for the AE process while the imaging element performs the imaging process as in the AF process. Thus, the AE process (contrast evaluation and lens drive processes) is performed until the reset process of the imaging element is started after a process of reading the imaging signal from the imaging element in the imaging process is completed.
In general, a sum time of several tens of milliseconds (ms) is necessary for the contrast evaluation and lens drive processes in the AF process. FIG. 16 is a timing chart illustrating processing timings when an imaging apparatus known in the related art captures an image. In the illustrated example, processing timings when a time necessary for the AF process is 100 ms and a time necessary to read a signal from the imaging element is 100 ms are illustrated. In this case, when the AF process is performed every time before shooting, a continuous shooting rate is about 5 shots/sec.
In addition, in order to improve the continuous shooting rate, a method of performing the AF process only once immediately before continuous shooting is started is known. FIG. 17 is a timing chart illustrating processing timings in the case in which the imaging apparatus known in the related art performs the AF process once immediately before continuous shooting is started when capturing an image. In the illustrated example, processing timings when a time necessary for the AF process is 100 ms and a time necessary to read a signal from the imaging element is 100 ms are illustrated. In this case, because the AF process is performed only once immediately before the continuous shooting is started, the continuous shooting rate is improved as compared to that of the case illustrated in FIG. 16.